A stripline circuit is often used as a transmission line structure of a radio-frequency (RF) transmission line. A transmission line can be used for communication between circuits such as between chips, racks, backplanes and other interconnection structures. A conventional stripline circuit comprises a generally flat strip metal signal conductor sandwiched between two parallel ground planes. A substrate supporting structure of insulating material includes a dielectric disposed between the signal conductor and the ground planes. The circuit has a characteristic impedance based on the signal conductor width and substrate height. The circuit has a dielectric loss approximately equal to the relative dielectric constant er of the substrate which is typically greater than one (1). Dielectric loss refers to the attenuation of a signal or a loss of energy due to the dissipation of energy in the dielectric. Dielectric loss tangent is the imaginary part of the dielectric constant, and determines the amount of loss of the medium. Similar to the dielectric constant, low loss tangents result in a “fast” substrate while large loss tangents result in a “slow” substrate. The stripline circuit has a propagation velocity Vp which is a parameter that characterizes the speed at which an electrical signal passes. Expressed as a percentage, it is the ratio of a signal's transmission speed compared to the speed of light in vacuum. Thus, the transmission of a signal in a vacuum would have a propagation velocity Vp of about 100. Propagation velocity Vp is proportional to the reciprocal of the square root of the relative dielectric constant er of the material through which that signal passes. Thus, a conventional stripline circuit having a substrate with a relative dielectric constant er greater than one (1) has a propagation velocity Vp less then the speed of light and an effective dielectric loss proportional to the relative dielectric constant er.
It is desirable for a transmission line structure to have a high propagation velocity Vp and low dielectric loss.